KC-500 amp repair question

[StevenP]

Hi - new member here. I'm repairing my Roland KC-500 amp. Not a great amp, but I still need it sometimes.

The audio signal appears at R228, but doesn't pass through the Q203 FET, and therefore doesn't proceed to the power amp.

Pin 2 of the FET is at ~-7VDC. I'm not familiar with the circuit that feeds that pin, but I understand it may be a (power-on) mute control circuit.

Any tips on my next steps would be appreciated. I've already verified that a signal injected at R205 passes through the power amp to the speakers just fine. Power supply and preamp are verified as working.

StevenP

[StevenP]

By the way, the pertinent schematic is the first page of the following PDF:

http://music-electronics-forum.com/a...00_pg-7-12.pdf

[mozwell]

This circuit is a turn on "de thump" circuit for the power amp.
If the voltage on D204 cathode is always -7V, we need to work out why.

Is there AC volts on the cathode of D207
Does C227 charge down below 0V
the voltage on cathode of D206 should start at -14.4V (or so) and slowly reduce down to -15V (or so)
Do C227 & C226 look ok
Do we have -15V on Q204 emitter, and +15V on one end of R223

short out R223, do you get sound now, if so, D204 & the fet Q203 are ok and the problem is earlier in the chain.
remove the short on R223 & short out D206, do you get sound on the speaker now, if so the transistor Q204 is ok, and diode D206 may be leaky and needing to be replaced (1N4148 will do)

If the thump is not too bad, you can always bridge out Q203

As these are simple low cost parts, we coudl always replace D207, D206, Q204, D204, Q203, although i never really like a scatter gun approach, best to work through the circuit & find out where we dont get what we expect to find, and then just fix that.

[StevenP]

short out R223, do you get sound now, if so, D204 & the fet Q203 are ok and the problem is earlier in the chain.
remove the short on R223 & short out D206, do you get sound on the speaker now, if so the transistor Q204 is ok, and diode D206 may be leaky and needing to be replaced (1N4148 will do)

If the thump is not too bad, you can always bridge out Q203

Hi mozwell,

Thank you for the very thoughtful and helpful reply! Much appreciated. You rock.

Yes, there is -15V on Q204 emitter, and +15V on one end of R223.

I shorted out R223, it resulted in sound. I removed that short, then shorted out D206. That also resulted in sound.

So, perhaps D206 is the culprit.

Addtl info: The anode of D207 is at ~-15VDC.

Thanks once again for your ideas so far.

- StevenP

[mozwell]

Steven
Excellent, we know the fet & control line for it are ok, we just need to get the mute line switching properly.
I woudl replace D206 & Q204, you can use 1N4148 for D206, & BC547 for Q204
Not sure of the pinouts between 2SC1815GR & BC547, so download datasheets & check it out.
You may also wish to replace the small electrolytics while you are at it, C227 & C227.
i woudl use 50V parts for both, use good quality types.
Sometimes these small electrolytics can go a bit leaky, which may rob enough base current from Q204 so that it doestn switch.

Come to think of it, replace the two electrolytics first & check if you get sound, if not, then replace the diode & transistor.

[data river]

KC-500 too harsh?

The Roland kc-500 I purchased on craigslist had a strange problem. Each channel sort of worked – but the amp would go into oscillation if any knob was turned above about 40%. What I found was that ALL the pots needed to be replaced. Good thing Roland includes the ALPS part numbers on the schematic. Not easy finding these but Mouser had all but the CH3 250K pots. For the tone 50K (type B taper) pots I used the dual gang pots ( and clipped off the extra leads) because that’s all I could get. Fortunately, the CH3 250k dual pot was ok – but not perfect. But ALPS doesn’t appear to have a part for this anymore.
So after I got that all taken care of, I started using the amp, but, with any higher volumes it sounds awfully harsh – like a weasel is living in there.
To fix this:
Roland put some equalization circuits in the power amp. After much calculation and messing around trying to flatten the response, I removed the following capacitors:
C225 (.0047uF)
C207 (.068uF)
C211 (1uF)
Roland uses these capacitors (and some resistors) in some equalization method which, frankly, doesn’t work well, especially with the horn. Removing these caps gives a straight eq which helps tremendously.
I also changed C213 from 330pF to 220pF, but this really didn’t have much of an effect.
But to completely fix the problem, I substituted a bullet tweeter with SPL 106db that is 100% better than the Fostex 025H27. I used a Pyramid TW47 that I had laying around – mainly because it fit in the existing hole for the Fostex horn – I only had to drill 4 more bolt holes. Previously, I used the TW47 in a Nemesis bass amp with 4 10” woofers, and it too sounded much better than the included horn (and it fit in the existing horn hole).
I also added some fiber fill to the cabinet – which was bare wood inside – that was a lot of labor that I am not sure helped, but it is generally recommended for sealed woofers to reduce internal noise.